How to Decline a Recommendation Request
If someone on LinkedIn or another social networking site asks for a recommendation, and you do not know the person very well, you can simply say, "I am sorry, but I do not feel that I know you well enough (or I have not worked with you long enough) to provide you with an accurate recommendation." If the person pursues the matter, simply explain that your integrity and professional brand is on the line with each recommendation you make, and you simply do not feel comfortable writing the recommendation.
If you know the person well, but do not feel you can give them a positive recommendation, you can simply say, "I do not feel I would be the best person to write you a recommendation," and perhaps provide a suggestion for another person they could ask.
Don't feel badly about saying no. It's better not to give a recommendation at all than it is to give a wishy-washy one or even a negative reference, which some people do without considering how it will impact the individual who asked them.
Here are sample letters and email messages you can use to decline the request.
Letter Samples Rejecting a Recommendation Request